John has made a very important point. I used slow growth Norway or Scots pine (deal?) instead of a lighter wood like cedar for my Gentry design Ruth. To keep the weight to about the same, I decreased the dimensions of the longitudinal stringers, gunwales and keel. The result was decreased stiffness, the boat flexed a bit up and down depending on where it was supported. I reduced this to a minimal amount by adding "stays" (as on a sailboat mast) in the form of 3mm Dyneema line on each side running from bow at the keel up through holes in the frames under the gunwales and down to the stern piece at the keel. Tightened each stay with a lanyard to keep the rocker as designed. I suspect that a deck would stiffen a hull considerably compared with an open boat.
Peter